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Competing ideas: A perfect t for theevidence

We’ve seen that evaluating an idea in science is not always a matter of one key ex-periment and a denitive result. Scientists often consider multiple ideas at once andtest those ideas in many different ways. This process generates multiple lines of evi-dence relevant to each idea. For example, two competing ideas about coral atoll for-mation (island subsidence vs.formation on debris-toppedunderwater mountains) wereevaluated based on multiplelines of evidence, includingobservations of reef and atollshapes, island geology, stud-ies of the distribution of plank-tonic debris, and reef drilling.Furthermore, different linesof evidence are assembledcumulatively over time as dif-ferent scientists work on theproblem and as new technolo-

gies are developed. Becauseof this, the evaluation of sci-entic ideas is provisional.Science is always willing toresurrect or reconsider an ideaif warranted by new evidence.

It’s no wonder then that the evaluation of scientic ideas is iterative and dependsupon interactions within the scientic community. Ideas that are accepted by thatcommunity are the best explanations we have so far for how the natural world works.But what makes one idea better than another? How do we judge the accuracy of anexplanation? The most important factors have to do with evidence—how well our actu-al observations t the expectations generated by the hypothesis or theory. The betterthe match, the more likely the hypothesis or theory is accurate.

• Scientists are more likely to trust ideas that more closely explain the ac-tual observations. For example, the theory of general relativity explains whyMercury’s orbit around the Sun shifts as much as it does with each lap (Mercury isclose enough to the Sun that it passes through the area where space-time is dim-pled by the Sun’s mass). Newtonian mechanics, on the other hand, suggests thatthis aberration in Mercury’s orbit should be much smaller than what we actuallyobserve. So general relativity more closely explains our observations of Mercury’sorbit than does Newtonian mechanics.

Mercury’s orbit around the sun shifts a bit with each lap, whichcan be explained by the theory of general relativity.

• Scientists are more likely to trust ideas that explain more disparate ob-servations. For example, many scientists in the 17th and 18th centuries were

Atoll satellite image by NASA/Goddard Space Flight Center; coral core sample photo by Jeff Anderson, Florida Keys NationalMarine Sanctuary

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puzzled by the presence of marine fossils high in the Alps of Europe. Some triedto explain their presence with a massive ood, but this didn’t address why thesefossils were of animals that had gone extinct. Other scientists suggested that sealevel had risen and dropped several times in the past, but had no explanation forthe height of the mountains. However, the theory of plate tectonics helped explainall these disparate observations (high mountains, uplifted chunks of the seaoor,and rocks so ancient that they contained the fossils of long extinct organisms)and many more, including the locations of volcanoes and earthquakes, the shapesof the continents, and huge rifts in the ocean’s oor.

• Scientists are more likely to trust ideas that explain observations thatwere previously inexplicable, unknown, or unexpected. For an example, seeRudolph Marcus’s story below …

JUMPING ELECTRONS!

As chemical reactions go, electron transfersmight seem to be minor players: an elec-tron jumps between molecules without evenbreaking a chemical bond. Nevertheless,such reactions are essential to life. Photo-synthesis, for example, depends on pass-ing electrons from one molecule to anotherto transfer energy from light to moleculesthat can be used by a cell. Some of thesereactions proceed at breakneck speeds, andothers are incredibly slow—but why shouldtwo reactions, both involving a single electron transfer, vary in speed?

In the 1950s, Rudolph Marcus and his colleagues developed a simple mathemati-cal explanation for how the rate of the reaction changes based on the amountof free energy absorbed or released by the system. The explanation t wellwith actual observations that had been made at the time, but it also generatedan unintuitive expectation—that some reactions, which release a lot of energy,should proceed surprisingly slowly, and should slow down as the energy releasedincreases. It was a bit like suggesting that for most ski slopes, a steeper inclinemeans faster speeds, but that on the very steepest slopes, skiers will slide downslowly! The expectation generated by Marcus’s idea was entirely unanticipated,but nevertheless, almost 25 years later, experiments conrmed the surprising ex-pectation, supporting the idea and winning Marcus the Nobel Prize.

What happens when science can’t immediately produce the evidence relevant to anidea? Absence of evidence isn’t evidence of absence. Science doesn’t reject an idea just because the relevant evidence isn’t readily available. Sometimes, we have to waitfor an event (e.g., the next solar eclipse), hope for a key discovery (e.g., transitionalwhale fossils in the deserts of Pakistan), or try to develop a new technology (e.g., amore powerful telescope), and until then, must suspend our judgment of an idea.

Rudolph Marcus

Rudolph Marcus image provided by the California Institute of Technology